If I have a button on a page (say sales order) and that button opens a suitelet in a popup (doable, fine), can anyone think of a way for a button on the suitelet to post back to the page that opened the suitelet? (the sales order)

You can use

window.opener.nlapi...

. I have an example script I can post later if you need more details.

I forgot about window.opener, great idea, thanks!